Most crypto trades do not involve USD because they are directly between one coin and another. Since each coin had a value in BTC, and BTC has a value in fiat, each altcoin has a value in fiat. That's why the new tax rules work (kind of).

Most people expect cryptocurrencies to behave like, well, currency. Isn’t it the case that with Forex trading you simply pay tax on total gains & losses, not every trade you made?

I don't know how US taxes work, but I have to pay tax when I do that. For example, I'm in Japan and if I buy CDN and then use it to buy GBP making a capital gain, I have to report it and pay taxes on it that year. Of course, if I sell the GBP and take a capital loss, I get to report that as well. There is actually a provision for allowing a "foreign reserve", so if you are doing a lot of business in one currency and don't want to convert to yen, you can "park" it without having to pay tax. Or at least that's my understanding of it. I have an accountant that takes care of details like that for good reason.